The use of probiotics to increase disease resistance and improve the overall health of terrestrial animals has long been established. However, their use and effectiveness in aquaculture environments has only recently been recognized.
Flavobacterium psychrophilum is the causative agent of coldwater disease (CWD) as well as rainbow trout fry syndrome (RTFS). While all salmonids are susceptible to F. psychrophilum, rainbow trout Oncorhynchus mykiss are especially affected, as observed through economic loss to aquaculture operations. Currently, no commercial vaccine exists for F. psychrophilum, leaving antibiotics as the primary form of treatment. While new antibiotic treatments such as florfenicol have recently been approved for control of CWD, there is continued concern over antibiotic use due to the potential development of bacterial resistance. Consequently, the use of a probiotic to decrease morbidity and mortality due to infectious disease, such as that caused by F. psychrophilum, would be useful as a potential method to reduce the use of traditional antibiotics in combating this and other microorganisms.
While the mechanisms allowing a particular probiotic to improve health are not always clear, the observed probiotic modes of action have been shown to include the production of inhibitory compounds, competitive exclusion, improvement of the immune response, and possible enhancement of water quality. While many definitions have been established to describe probiotics, several characteristics, including survival and colonization of the gastrointestinal tract are noted as important characteristics of probiotic selection. Additionally, an effective probiotic should not directly cause mortality in the animal that it is intended to be used in. Other desirable characteristics of effective probiotics for use in aquatic animals include non-pathogenicity to both the host and humans, resistance to bile salts, pH tolerance, antagonism towards the pathogen of interest, the ability to adhere to and colonize the intestine of the host, strong growth characteristics, and autochthonous to the host or its environment.